As is known, relative to a reciprocal engine, a gas turbine is compact and lightweight and can provide high power.
A gas turbine, e.g., a balanced pressure combustion type gas turbine, normally comprises a cylindrical casing and a rotating shaft which is rotatably arranged in the casing. A compressor and a power turbine are formed between two ends of the rotating shaft and the casing. A plurality of combustors are arranged between the compressor and the power turbine, and pressure in the combustors is increased by high-pressure air compressed by the compressor. In this state, fuel is injected to the combustor and is combusted. A high-pressure, high-temperature gas, generated by combustion, is guided to the power turbine and is expanded in volume, thereby obtaining power for rotating the rotating shaft.
The compressor has an axial flow arrangement, where rotor blades fixed to the rotating shaft and guide vanes fixed to the casing are alternately arranged along the axial direction of the rotating shaft. In the power turbine, rotor blades fixed to the rotating shaft and nozzle vanes fixed to the casing are alternately arranged along the axial direction of the rotating shaft.
In the gas turbine with the above arrangement, as a most effective means for improving a gas turbine efficiency, a gas temperature at the entrance of the power turbine is increased. However, the maximum permissible temperature of the metal material constituting the power turbine is normally about 850.degree. C. Therefore, in order to increase the gas temperature beyond the permissible temperature, members constituting the power turbine, in particular, blades, must be cooled with high efficiency.
In a conventional gas turbine using clean fuel such as petroleum, LNG, or the like, the blade is cooled by a cooling method combining a convection cooling method, wherein the blade is cooled from inside, and a film cooling method, wherein cooling air is ejected from a plurality of portions of the blade to cool the blade. Cooling air ejection holes are formed at high density on a portion, (e.g., a leading edge portion) of the blade, which becomes very high in temperature, thus providing a so-called shower head structure.
In recent years, a high-efficiency coal gasification combined power generation system using dirty fuel such as coal gasification fuel has been developed. In this system, a gas temperature at the turbine entrance must be increased beyond 1,300.degree. C. in order to improve a plant efficiency. However, when the turbine is operated under the high-temperature condition, coal ash may become attached to the blade surface, or the blade surface may be corroded by the ash. For this reason, cooling air ejection holes which are open to the blade surface may often clog. Therefore, in this system, the normal film cooling method cannot be effectively utilized exclusively.
Accordingly, it is difficult to realize a high-efficiency gas turbine using dirty fuel, unless the blade is satisfactorily cooled not only by the film cooling method but also by other means.